The present invention is directed to laminated products such as diapers, incontinence garments, surgical gowns, face masks, and the like. Specifically, the present invention is directed to laminated products which comprise a first substrate, a second substrate, and an adhesive composition. The first and second substrates are dissimilar materials or materials that are non-ultrasonically bondable using conventional means. The adhesive composition comprises a specific mixture of atactic and isotactic polymers such that it allows for and aids in the ultrasonic bonding of dissimilar materials or non-bondable materials.
People rely on disposable absorbent articles to make their lives easier. Disposable absorbent articles, such as adult incontinence garments and diapers, are generally manufactured by combining several components. These components typically include a liquid-permeable topsheet, a liquid-impermeable backsheet attached to the topsheet, and an absorbent core located between the topsheet and the backsheet. When the disposable article is worn, the liquid-permeable topsheet is positioned next to the body of the wearer. The topsheet allows passage of bodily fluids into the absorbent core. The liquid-impermeable backsheet helps prevent leakage of fluids held in the absorbent core. The absorbent core generally is designed to have desirable physical properties, such as, for example, a high absorbent capacity and high absorption rate, so that bodily fluids can be transported from the skin of the wearer into the disposable absorbent article.
Frequently, one or more components of a disposable absorbent article are first adhesively, and then ultrasonically bonded together to ensure adequate strength of the resulting bond. For example, conventional hot melt adhesives have been used to first bond individual layers of the absorbent article, such as the topsheet (also known as, for example, the body-side liner) and backsheet (also known as, for example, the outer cover), together. Conventional hot melt adhesives have also been used to bond discrete pieces, such as fasteners and leg elastics, to the article. In many cases, the bonding together of two components (whether for a permanent-type bond or simply for holding components in place during the manufacturing process) forms a laminated structure in which adhesive is sandwiched between materials (such as layers of polymer film and/or layers of woven or nonwoven fabrics) that make up the components being bonded together. Once the laminated structure is formed, the laminate will typically undergo an ultrasonic bonding process to impart increased strength in the bonded area of the laminate.
Conventional hot melt adhesives generally utilized in adhesive bonding of materials in laminated absorbent products generally comprise several components including: (1) one or more polymers to provide cohesive strength; (2) a resin or analogous material to provide adhesive strength; (3) waxes, plasticizers, or other materials to modify viscosity; and (4) other additives such as antioxidants and stabilizers. Conventional hot melt adhesives are well known in the industry to those skilled in the art.
Ultrasonic bonding is a conventional bonding technique wherein materials are exposed to a high frequency vibration which results in a heating, melting, and flowing of the polymeric materials into each other to form a mechanical and/or chemical bond. Although commonly utilized in the production of laminated absorbent articles, ultrasonic bonding can become problematic when dissimilar or non-bondable materials are utilized. That is, the ultrasonic bonding of polymer based films or non-woven materials that have melting points that vary greatly from each other can be very difficult, or even impossible to accomplish as a polymer with a lower melt temperature that will soften and dissipate away from the bonding zone before the polymer material with the higher melting point will soften. Additionally, the ultrasonic bonding of non-thermoplastic materials, such as the ultrasonic bonding of two cellulosic materials (e.g., tissues) together, is also very difficult, if not impossible as the materials do not melt and flow together. Although it may be possible to achieve some ultrasonic bonding in these situations, any resulting bond is typically weak and unreliable.
Further, ultrasonic bonding of dissimilar or non-bondable materials can become problematic in the presence of conventional hot melt adhesive materials. For example, during ultrasonic bonding, the adhesive composition can result in bleedthrough of the adhesive through one or both of the materials. This bleedthrough can result in at least three significant problems. First, such bleedthrough can result in a discolored end product. Such discoloration, although typically not affecting product performance, is not desirable for consumers who prefer white, uncolored, clean-looking products. Second, the bleedthrough on the end product can result in a tacky product which sticks to skin upon use, which is not desirable for consumers. Third, the bleedthrough can result in an adhesive residue build-up on the ultrasonic bonding equipment and other equipment used in the manufacturing process. Such an adhesive build-up can result in the need for frequent cleaning of the machinery, which increases costs as numerous contaminants can adhere to, and build up on, the adhesive. Additionally, the adhesive build-up on the machinery can result in the adhesive composition being deposited on absorbent products in unintended areas.
Additionally, conventional hot melt adhesive compositions exhibit viscous flow behavior with much lower softening points. These characteristics may result in the creation of a heat sink during ultrasonic bonding. When a heat sink is created, a high percentage of the ultrasonic energy of the system is used for re-melting the adhesive in the bonded area, which leads to bleedthrough under pressure and heat. As such, less ultrasonic energy remains to melt the thermoplastic materials and perform the ultrasonic bond between the materials. The re-melting of the adhesive is not an optimal use of ultrasonic energy as an adhesively bonded joint is typically not as strong as an ultrasonically bonded joint as the bond strength is limited to the cohesive strength of the adhesive. Also, cohesive strength may vary significantly with temperature and, in the case of absorbent care products, body heat may be sufficient to weaken the strength of the adhesive bond to the point of failure.
Based on the foregoing, there is a need for a hot melt adhesive composition that can be utilized as an adhesive between dissimilar or non-ultrasonically bondable materials that will result in an improved ultrasonic bond between the materials. It would also be beneficial if the hot melt adhesive did not result in significant bleedthrough onto the product and did not significantly foul the machinery utilized in the manufacturing process.